1. Field of the Invention
The present invention relates to assembling an integrated circuit package. More specifically, the invention relates to facilitating heat and force transfer between the integrated circuit package after a silicon layer has been bonded to the substrate.
2. Description of the Related Art
Digital circuits, no matter how complex, are composed of a small group of identical building blocks. These blocks can be gates or special circuits or other structures for which gates are less suitable. But the majority of digital circuits are composed of gates or combinations of gates. Gates are combinations of high-speed electronic switches such as transistors.
A microprocessor is a central processing unit of a computer or other device using thousands (or millions) of gates, flip-flops and memory cells. Flip-flops and memory cells are modified versions of basic logic gates.
It is known to manufacture an integrated circuit using conductors separated by a semiconductor. Circuits are fabricated on a semiconductor by selectively altering the conductivity of the semiconductor material. Various conductivity levels correspond to elements of a transistor, diode, resistor, or small capacitor. Individual components such as transistors, diodes, resistors, and small capacitors are formed on small chips of silicon. These individual components are interconnected by wiring patterns (typically aluminum, copper or gold).
An integrated circuit is then included in a larger structure, known as integrated circuit package, that provides electrical connections between the integrated circuit and the next level assembly. The integrated circuit package also serves structural functions. Integrated circuit packages are then mounted on printed (or wired) circuit boards which are used to assemble electronic systems such as personal computers and other data processing equipment.
It is known to manufacture an integrated circuit package using a layer of silicon and a layer of a substrate. The substrate layer can be ceramic or another material with the necessary electrical insulating properties, such as a ceramic. Heat is applied during the manufacturing process to bond the silicon layer to the substrate layer. Uneven cooling of the silicon and substrate layers (sometimes referred to as the xe2x80x9cpackagexe2x80x9d) could produce failures in the package. Uniform cooling minimizes the number of failures in the package.
After bonding the silicon layer to the substrate layer a heat spreader (sometimes referred to as a xe2x80x9cthermal lidxe2x80x9d or simply a xe2x80x9clidxe2x80x9d) is attached to the package. The thermal lid serves to conduct heat from the integrated circuit package to the environment and thus facilitates even cooling. The lid is typically formed from a metal due to the high thermal conductivity of metals. Typically, neither the thermal lid nor the silicon surface are sufficiently flat to provide an efficient heat exchange interface. Thus, imperfections in the surface of the thermal lid and the surface of the silicon prevent complete surface contact between the surface of the silicon and the surface of the thermal lid. The incomplete surface contact is an impediment to heat transfer which in turn causes failures of the package.
The lid can be used in conjunction with a heat sink. The heat sink is provided with fins or other external surfaces to increase contact with ambient air. The increased contact with the ambient air further facilitates heat transfer.
The lid also serves to promote even transfer of forces to the package. Even transfer of force to the package prevents force concentrations on the silicon layer, substrate or in some circumstances the printed circuit board. Even force transfer also reduces failures of the package.
To facilitate surface contact between the thermal lid and the silicon surface a thermal interface (sometimes referred to as a xe2x80x9cdie interfacexe2x80x9d) is employed. The die interface can be applied to the surface of the silicon before the thermal lid is attached. The die interface is not necessarily a solid and can conform to imperfections in the surface of the silicon. Similarly, the die interface can conform to imperfections in the surface of the thermal lid. Thus, using a die interface increases the surface contact between the silicon and the thermal lid and promotes heat transfer.
The increased surface contact between the silicon surface and lid has an additional benefit. When the lid is applied to the silicon layer a force is transferred. If the force is not uniformly transferred, imperfections in the silicon surface can result. Failures of the silicon surface can result in rejected packages or later failures.
When a heat sink is employed it is also known to utilize a heat sink interface. Similar in material characteristic to a die interface, a heat sink interface is not necessarily a solid but can also be a viscous liquid. Similar in function to a die interface, a heat sink interface also improves heat transfer properties by improving surface contact between the heat sink and the lid. Similar to the die interface the heat sink interface improves force transfer by increasing surface contact between the heat sink and the lid.
An example of a material that is suitable for use as an interface is manufactured by Thermagon of Cleveland, Ohio. This specific material, referred to as T-lma-60, has suitable thermal conductive properties and can be used as a thermal interface. T-lma-60 can have more than one layer and is a thermal conductive structure phase change material. T-lma-60 changes phase from solid to liquid at approximately 60xc2x0 C. A thermal interface, such as T-lma-60 or other, can have a plurality of layers. For example, a thermal interface such as T-lma-60 can have three layers, one of which can be a metallic central layer.
FIG. 1A depicts substrate 120 adjacent to printed circuit board (sometimes referred to as xe2x80x9cpcbxe2x80x9d) 110. Silicon die 130 is bonded to substrate 120 as previously discussed. Die interface 150 is typically a non-solid used to facilitate heat transfer between silicon die 130 and lid 140. Lid 140 contacts heat sink interface 160 as shown. Heat sink interface 160 contacts heat sink 170 as shown. Lid interface 180 is used to facilitate heat transfer between silicon die 130 and substrate 120. FIG. 1B depicts a thermal lid with a cavity depth of zero. As depicted in FIG. 1B, lid interface 180 is not typically used for applications having a zero cavity thermal lid due to the lack of surface contact between thermal lid 140 and substrate 120.
FIG. 2A depicts the logical steps of placing die interface 150 on silicon die 130. As shown in FIG. 2A, the method begins with start 210. From start 210 the logical steps include providing substrate, providing silicon die 220 and providing thermal lid 240. After providing silicon die 220 and providing substrate 230 the silicon die 130 and substrate 120 are bonded, 250. Provide thermal lid 240 is shown occurring prior to bonding (250) silicon die 130 to substrate 120 but can occur later. After providing thermal lid 240 die interface is placed (260) on silicon die 130. After die interface is placed on silicon die 130 lid interface 180 is placed (265) on substrate 120. After lid interface is placed (265) on substrate, thermal lid 140 is placed on die interface 150 and lid interface 180. In one method, after the thermal lid and thermal interface are placed on the silicon layer 270, the process ends 295.
Another embodiment represented in FIG. 2B. In the embodiment represented in FIG. 2B, heat sink 170 is provided, 255. When a heat sink is provided heat sink interface 160 is also provided, 265. As shown in FIG. 2B, heat sink interface 160 is placed (280) on thermal lid 140 after the thermal lid is placed (270) on the silicon die. As further shown in FIG. 2B, heat sink 170 is placed (290) on thermal interface 160 after heat sink interface 160 is placed (280) on the thermal lid.
Although FIG. 2B depicts providing heat sink 170 and heat sink interface 160 after bonding (250) heat sink 170 and heat sink interface 160 can be provided earlier or later in the process. For example, referring to FIG. 2C, providing heat sink 255 and providing the heat sink interface 265 occur after placing (260) die interface on silicon die.
Each of the following components contribute to the total thermal resistance of the package: heat sink, heat sink interface, thermal lid, die interface, silicon die and substrate. Thus heat transfer is constrained by the number of components and the thermal conductivity and physical characteristics (such as thickness) of those components. What is needed is a method of improving the thermal conductivity of these components, or reducing the number (or thickness) of the components.
In accordance with the present invention, a method is described which facilitates heat transfer from a silicon die after the silicon die is bonded to a substrate. A thermal conductor, (e.g. a heat exchanger, heat sink, thermally conductive lid or other means) is placed on the silicon layer after the silicon layer has been bonded to the substrate layer. A spacer is used between the substrate and the thermal conductor. The spacer can facilitate heat transfer from the die. The spacer facilitates force transfer from the thermal conductor to the die. In an embodiment, the thermal conductor can be removed and a second thermal conductor used to further facilitate heat transfer. In an enablement, a heat sink and heat sink interface are provided and further facilitate heat transfer from the package.
The specification also teaches an integrated circuit package manufactured by the method taught. The specification also teaches a computer system including an integrated circuit package manufactured by the method taught.
The foregoing is a summary and this contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting.